1. Field of the Invention
The present invention relates to magnetic data storage systems generally and, more specifically, to low noise balanced preamplifiers useful for read and control channels in magnetic storage systems.
2. Description of the Related Art
A popular method of magnetic data storage utilizes magnetoresistive (MR) heads to store and recover data on a magnetic data storage medium such as a magnetic disk. An MR head employs an MR element that changes in resistivity with changing magnetic flux from data patterns on an adjacent magnetic disk surface. A bias current having a constant value is passed through the MR element, and the change in resistivity is measured by sensing the change in voltage across the MR head.
Preamplifier circuits that sense signals directly from a MR head commonly include differential inputs and differential outputs. A preamplifier is typically embodied in an integrated circuit, possibly with other preamplifiers and other circuitry such as write drivers, and is collocated with, or is positioned very near, the MR head. While there are a wide variety of balanced amplifier circuit topologies, most include an input stage driving two load resistors and symmetrical transistors for splitting current between the load resistors to generate an amplified output signal. Usually, the output signal is a differential voltage taken as the difference in the voltage drops across the load resistors; in this manner, large variations in output voltages may be achieved with extremely small input voltage differentials.
New magnetic storage technologies require sensing how high the MR head is above the rotating magnetic disk (referred to herein as fly-height control) in addition to reading data from the disk. To do so, multiple control systems are connected to a fly-height sensing head, co-located with the MR head, to read how high the head is from the magnetic disk media during the reading of information therefrom. Previously the frequency bands of two of the control systems, referred to as DC sense and thermal asperity (TA) sense, did not overlap so interaction between the DC and TA sense systems was typically not a factor. However, new magnetic storage systems utilize overlapping frequency bands for fly-height control. Existing preamplifier designs have too low an input impedance to permit connecting the two sense paths to the same sense head because the frequency response of the preamplifier is not sufficiently flat over the bandwidth of the signals being amplified by the preamplifier. Thus, a new preamplifier design is desired to provide a substantially flat frequency response over the overlapping frequency bands of the DC and TA sense systems.